This invention relates to a sulfonic acid bath for tin-lead alloy plating capable of giving a deposit of stabilized tin-lead alloy composition.
For tin-lead alloy plating the use of commonly employed borofluoride baths has been subject to varied limitations due to the necessity of disposing of the resulting fluorine-containing wastewater. From this viewpoint tin-lead alloy plating baths using organic sulfonic acids of relatively low toxicity have recently been proposed. For example, Japanese Patent Application Nos. 176365/1982 and 55190/1983 disclosed that light-grayish, uniform, fine-grained electroplated coatings of tin-lead alloy could be obtained by adding to an organic sulfonic acid bath a nonionic surface active agent, such as an adduct of styrenated phenol with an alkylene oxide (e.g., polyoxyethylene tristyrylphenyl ether, POETSPE) and an additive, such as a certain sulfanilic acid [e.g., N-(3-hydroxybutylidene)-p.dbd.sulfanilic acid, HBPSA] and/or a triazine [e.g., 2,4-diamino-6.dbd.{2'-alkylimidazolyl(1')}ethyl-1,3,5-triazine, DAAIMET].
Tin-lead alloy (generally known as solder) plating is used extensively in light electric and electronic industries for joining metallic surfaces of components. For applications wherein occurrence of whiskers is undesirable, solder deposits containing from a few % to 20% of lead are applied. For applications wherein resistance to corrosion is required, solder deposits containing from 70% to 80% of lead are applied. Further, in fabricating printed-circuit boards, 60/40 eutectic solder deposits are applied as an etching resist.
Thus, since deposits having various compositions are required for tin-lead alloy plating according to their applications, it is ideal to always obtain deposits having a constant composition even if the current density changes from low to high.
For example, the printed-circuit boards with the tin-lead alloy are usually subjected to fusing, a treatment for removing overhangs and enhancing the solderability. The treatment, however, will give uneven, rough treated surfaces if the deposit produced by electroplating on the surface regions of the printed-circuit board is dissimilar in composition to that formed in through-hole plating with consequent difference in melting point between the two deposits. Therefore, in plating printed-circuit boards with a tin-lead alloy, it is necessary to assure deposition of a uniform composition throughout the surface regions and holes of the boards.
For the tin-lead alloy plating of printed-circuit boards semibright plating techniques are in wide use because in many cases brightness is not the first consideration and because the techniques permit smooth and even electroplating with good fusibility.
The plating baths described in the above-mentioned patent applications produce tin-lead alloy plates with fairly improved throwing power and fusing property. Under low current density conditions, however, they tend to increase the lead contents in the resulting deposits of tin-lead alloy, rendering it impossible to form plated coatings of the desired Sn/Pb ratio. In order to ensure high reliability required of printed-circuit boards, it is imperative that the Sn/Pb ratio in the deposits be stable, the deposits be improved in the fusing property and in stability against heat to be applied in subsequent process steps, and the plating bath be easy to control.
In view of the foregoing, we have investigated various addition agents. As a result, it has now been found that a certain group of guanamine compounds give tin-lead alloy plated coatings having a constant Sn/Pb ratio, under not only low current density conditions but even high current density conditions, the Sn/Pb ratio being substantially the same as that of the plating bath. It has also been found that these guanamine compounds yield plates possessing good throwing power, fusing property, and heat resistance without the addition of any such nonionic surface active agent or additive as referred to in the cited patent applications.